The present invention concerns a grinding aid consisting of fluoride-containing inorganic compounds for use in an abrasive material comprising abrasive grains and a grinding aid, a method of producing it, use of it as well as an abrasive material containing it.
It is known to use cryolite as a grinding aid in the abrasive composition in abrasive materials, such as sandpaper, emery cloth and grinding wheels, cf. e.g. KIRKOTHMER: ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY, Third Edition, Vol. 10, 1980, line 672, and the mention of the prior art in WO 94/02562.
WO 94/02562 concerns abrasive grains having an outer surface to which a grinding aid material is bonded via interparticle attraction. A large number of materials, inter alia cryolite, including synthetic cryolite, are mentioned as a grinding aid material.
U.S. Pat. No. 5,078,753 concerns an abrasive material having on the abrading face, in addition to abrasive grains, erodable aggregates consisting of a finely ground filler and a resinous binder. The filler may be calcium carbonate, talc, glass, sodium sulfate, cryolite, fluoroborates, etc. Example V uses inter alia sodium cryolite.
EP Patent Application No. 8 868 describes abrasive materials consisting of brittle agglomerates of abrasive grains bonded in an inorganic matrix. The matrix may be based on natural cryolite, e.g. type S from KRYOLITSELSKABET .O slashed.RESUND A/S Danmark. These abrasive materials may be produced by melting natural cryolite, adding the abrasive grains, cooling and crushing the cooled material.
Grinding aids are used, because they have a positive influence on the utility properties of the abrasive materials, inter alia grinding rate and product life. No specific knowledge is available about the mechanism of the product improvement obtained. However, it is assumed that the effect of using cryolite as a grinding aid may be due to the circumstance that the chipping created by grinding locally takes place at very high temperatures, and that the cryolite acts as a flux in a melt produced by the chipping, whereby the viscosity of the melt is reduced. However, the effect of the cryolite may also be due to the circumstance that any tendency to block the gaps between the abrasive grains is diminished.
For many years KRYOLITSELSKABET .O slashed.RESUND A/S Danmark has supplied a ground natural cryolite for this purpose; thus, the company has supplied natural cryolite ground to -325 mesh, i.e. less than 45 micrometers, to the grinding disc industry.
After the termination of the mining of cryolite in Greenland, natural cryolite is today no longer available in the market. A few producers (from Russia) sell grades which they call natural cryolite, but tests have shown that it is precipitated cryolite types. There are deposits of natural cryolite in Russia, but, as far as is known, they are not suitable for working by flotation. Possibly, the Russian "natural cryolite" is made by extraction of such a deposit with subsequent precipitation.
Thus, in the mid-1990s the cryolite types available in the market are generally synthetic, i.e. produced by a wet precipitation process, and it has been found that none of these commercially available cryolite types are suitable as a grinding aid in abrasive compositions.
The difficulties involved by the use of synthetic cryolite appear to be of two types:
Firstly, there is an aesthetic problem, since the use of synthetic cryolite as a grinding aid results in undesirable stripes in the coating on sandpaper, especially on the finer types. Further, it has been observed that the synthetic cryolite absorbs more binder than the natural cryolite on both fine and coarse sandpaper types.
Secondly, there are productionally unacceptable variations in the course of a production of e.g. sandpaper, there being thus an increasing particle separation and particle agglomeration tendency during the course of the production.
Finally, it has been reported that users of the abrasive materials have experienced unspecified performance problems.
Through detailed studies, the applicants have been able to demonstrate both chemical and morphological differences between natural cryolite and synthetic cryolite, i.e. cryolite produced by a wet precipitation process. The chemical differences have the result that melting point and melt composition will be different for the two types. This may be of importance e.g. to the function as a flux during the grinding process.
The chemical composition of various cryolite types is approximately as follows:
______________________________________ F (%) Al (%) Na (%) ______________________________________ Materials from Kryolitselskabet .O slashed.resund: Natural cryolite E 54.3 12.9 32.9 Natural cryolite S 54.0 12.3 31.5 Russian materials: Synthetic neutral 53.5-54.5 14-15 28-29 Synthetic acid 55.5 16.7 24.8 Hungarian material: Synthetic 53.2 13.3 ______________________________________ where "%" means % by weight.
Pure, neutral cryolite has the composition 3 NaF.1 AlF.sub.3, i.e. an NaF/AlF.sub.3 molar ratio of 3.
A cryolite having an excess of AlF.sub.3, i.e. having an NaF/AlF.sub.3 molar ratio below 3, is called acid cryolite, while a cryolite having an excess of NaF, i.e. having an NaF/AlF.sub.3 molar ratio above 3, is called a basic cryolite. The natural cryolite is neutral, while all the synthetic cryolites are more or less acid. The so-called "natural cryolite" from Russia has the same composition as synthetic cryolite and a particle shape and size which can just have been obtained by precipitation from an aqueous solution.
Microscopic studies of various cryolite types show considerable morphological differences. Generally, all the studied synthetic cryolites consist of extremely fine crystals of a quite regular size, typical particle size:
1-2 micrometers, which have agglomerated to particles of a size less than about 25 micrometers, while the natural cryolites are very coarse-grained individual crystals or crystal fragments of widely different particle sizes and with irregularly shaped particles. In other respects, inter alia flowability and dusting and lumping ability, there are just minor differences between the various cryolite types.
Thus, it will be seen that the cryolite types suitable as grinding aids are chemically characterized by being neutral, while the unsuitable grades have been more or less acid.
The problem underlying the invention is to provide a grinding aid which is suitable as substitute for the finely ground, naturally occurring, neutral cryolite used previously, and which does not exhibit the above-mentioned drawbacks associated with the use of synthetic cryolite, a method of producing it, use of it for producing abrasive materials, and abrasive materials containing it.